Not applicable.
This invention relates to an improved industrial furnace and burner system used to melt and refine glass. More specifically, this invention relates to system of interacting fuel-rich and fuel-lean flames generated by at least a pair of oxy-fuel burners to enhance heat transfer to the material to be melted, glass melt or batch, by maximizing the flame area and temperature in contact with the solid batch or liquid melt interface.
Traditionally, glass production furnaces have relied on air-fuel burners as the primary heat source. In order to achieve reasonable overall process thermal efficiencies, either regenerative or recuperative heat recovery systems are typically used to preheat the air feed. A regenerative heat recovery system pre-heats the air feed by alternately using the hot furnace off-gases to heat refractory and then using this hot refractory to preheat the air feed. A recuperative waste heat recovery system achieves the same goal using a shell and tube heat exchanger to transfer heat from the hot furnace gases to the air feed through a metallic tube wall. The air-based combustion systems have two major disadvantages.
First, the predominate mode for heat transfer with air fired system is conduction from the hot combustion gases to the refractory walls and roof and radiation from the refractory surfaces to the glass melt. This heat transfer rate is severely limited by the maximum operating temperature of the furnace refractory, low emissivity of the glass melt, and low thermal conductivity of the glass melt. Bubblers are typically used to somewhat increase the glass melt circulation and increase the effective conductivity of the glass melt. In addition, electrical resistance heating can be used to significantly increase the glass melting rate with a substantial increase in operating costs. However, the maximum roof temperature and low glass melt emissivity significantly limits the specific glass melting rate, typically measured as tons of glass/ft2/day. This low specific melting rate significantly increases the furnace capital cost.
The second problem with air based combustion systems is that recuperative and regenerative heat recovery systems are expensive and typically contaminants in the furnace off-gas plug and degrade recuperators and regenerators over time. As a result, recuperative and regenerative heat recovery systems significantly increase capital and maintenance costs and degrade furnace performance over time.
Oxy-fuel burners and electrical resistance heating have been used for many years to compensate for the adverse effect of the degradation in productivity of recuperative and regenerative over time. Oxy-fuel burners have had two traditional problems. First, the use of substantially pure oxygen increases the furnace operating cost. Second, oxy-fuel burners have a shorter and hotter flame. This shorter and hotter flame can cause localized overheating of refractory in the region of the burner. In addition, the short flame of oxy-fuel burners makes it difficult to heat the glass melt near the center of large furnaces.
U.S. Pat. No. 6,237,369 to LeBlanc et al. discloses controlling the velocity of gaseous fuel and oxygen in an oxy-fuel burner mounted in the roof of a glass melting furnace for increasing the melting rate of the glass.
U.S. Pat. No. 5,139,558 to Lauwers discloses the use of oxy-fuel burners located on the furnace roof and aimed at the interface between the batch and the melt in order to increase the melting rate of the glass and to prevent batch materials from entering the upstream zone. However, because the furnace roof to glass melt distance of often greater than typical oxy-fuel flame length, the thermal efficiency of the process is not adequate for use outside of regenerative or recuperative furnaces.
U.S. Pat. Nos. 5,346,524 and 5,643,348 to Shamp et al. attempt to overcome this problem by injecting oxygen and fuel at separate points and producing a large combustion xe2x80x9cflame cloudxe2x80x9d in the center of the furnace. This approach eliminates the oxy-fuel flame length limitation, but suffers from a safety problem. There is no apparent reliable method to heat the fuel and oxidant prior to mixing to maintain reliable ignition of the fuel and oxidant of this process. Premature mixing of the fuel and oxidant could lead to explosions.
In U.S. Pat. No. 5,387,100 to Kobayshi the use of rich and lean fuel streams to reduce NOx emissions is disclosed, however, there is no teaching regarding the interacting of fuel-rich and fuel-lean flames to increase the transfer of heat to the batch or melt.
Therefore, there is a clear and long standing need for a burner and furnace design that increases the oxy-fuel flame length, increases the heat transfer from the flame to the solid glass forming components and glass melt, while decreasing the heat transfer to the furnace roof or side-walls.
Accordingly, the present invention increases the amount of heat transferred to the melt in a furnace by using at least one pair of burners designed to each emit either a fuel-rich and fuel-lean flame which flames are directed to interact in the vicinity of the melt or batch material.
Furthermore, in the present invention fuel-rich and fuel-lean flames are emitted with a lower thermal energy and higher chemical energy than conventional oxy-fuel flames to transfer more heat to the batch materials or melt and direct heat away from the roof or side-walls of the furnace.
Reaction 1 illustrates conventional O2-fuel combustion.
CaHb+c[a+b/4]O2xe2x86x92aCO2+b/2 H2O+[c-1][a+b/4]O2xe2x80x83xe2x80x83[1]
Where, CaHb represents a gaseous or liquid fuel and c is the fraction of the stoichiometric oxygen addition. Typically c ranges from 0.8 to 1.3 and preferably from 0.9 to 1.2 depending on the quantity of air ingression into the glass melting furnace. Values of c less than unity may be used to increase the flame luminosity. However, a secondary oxygen source is required in order to consume all the carbon monoxide and hydrogen before discharging the combustion gases to the atmosphere. Combustion with substantially pure oxygen produces a very hot and short flame relative to air-fuel combustion. This very short and very hot flame results in very high heating rates in the region of the burner, which can cause excessive heating of the burner block or furnace refractory rather than heating of the glass forming material, particularly toward the center of the furnace. This invention overcomes this problem by producing fuel-rich and fuel-lean flame pairs that contain less thermal energy and much more chemical potential energy than conventional oxy-fuel flames.
Reaction 2 illustrates the stoiciometry for a fuel-rich flame.
zCaHb+[axz+a/2(1xe2x88x92x)z+byz/4]O2xe2x86x92axzCO2+a(1xe2x88x92x)zCO+(byz/2)H2O+bz/2 [1xe2x88x92y]H2xe2x80x83xe2x80x83[2]
The actual combustion process would contain many more species. In Reaction [2], the CO and H2 products can be further oxidized to CO2 and H2O to subsequently release additional thermal energy. Reaction [2] is intended only to illustrate the overall stoichiometry of this combustion process. Where, x and y are the fraction of the carbon and hydrogen, respectively, in the liquid or gaseous fuel that are fully oxidized in the fuel-rich flame. Typically, x and y would be in the range of 0.1 to 0.8. In addition, z is fraction of the gaseous or liquid fuel that is used in the fuel-rich flame. The balance of the fuel would be used in the fuel-lean flame.
Since both the fuel-rich and fuel-lean flames have a much lower temperatures than conventional O2-fuel flames, much less heat is transferred to the burner block and refractory in the region of the burner. These fuel-rich and fuel-lean flames are advantageously directed toward a common area near the interface of the batch material or the melt. As the flames interact, combustion can be completed near the batch material or batch/melt interface, which results in higher temperature combustion gases in contact with batch materials or the melt and, therefore, more efficient heat transfer to the batch material or melt.
Furthermore, by initiating combustion at the burner block and completing a substantial portion of the combustion at a secondary combustion zone in the vicinity of the batch or melt various dissociation reactions take place. The dissociated radicals impinge on the melt surface and recombine exothermically giving up additional heat to the melt.
A further object of the present invention is an increased uniformity of the heat distribution at the melt or batch surface.
A still further object of the present invention is the reduced exposure to heat of the burner assembly due to the lower temperature of the flame near the burner. Such a reduction in heat at the burner should result in an increased operating life for the burners.
Yet another advantage of the present invention is the lower fuel consumption and lower oxidant (O2) consumption per tons of melt processed by the furnace.
In one embodiment axisymmetric burners are placed in the roof of a furnace according to the present invention.
In another embodiment non-axisymmetric or axisymmetric burners are placed in the side-walls of a furnace according to the present invention.
In yet another embodiment of the present invention curved burners are placed in either the side-wall or roof of a furnace according to the present invention providing a means for altering the combustion intensity profile inside the furnace.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.